Seven Colours of the Rainbow

A rainbow is an optical phenomenon caused by refraction, internal reflection and dispersion of light in water droplets resulting in a continuous spectrum of light appearing in the sky.

The rainbow takes the form of a multicoloured circular arc.

Rainbows caused by sunlight always appear in the section of sky directly opposite the Sun.

Rainbows can be caused by many forms of airborne water.

These include not only rain, but also mist, spray, and airborne dew.

Jean-Léon Gérôme | La République, 1848 | Musée des Beaux-Arts de la Ville de Paris, Petit Palais

Rainbows can be full circles.

However, the observer normally sees only an arc formed by illuminated droplets above the ground, and centered on a line from the Sun to the observer's eye.

In a primary rainbow, the arc shows red on the outer part and violet on the inner side.

This rainbow is caused by light being refracted when entering a droplet of water, then reflected inside on the back of the droplet and refracted again when leaving it.

Alexander Shevelev

In a double rainbow, a second arc is seen outside the primary arc, and has the order of its colours reversed, with red on the inner side of the arc. This is caused by the light being reflected twice on the inside of the droplet before leaving it.

Newton originally (1672) named only five primary colours: red, yellow, green, blue and violet.

Later he included orange and indigo, giving giving seven colours by analogy to the number of notes in a musical scale.

Newton's first colours	Red		Yellow	Green	Blue		Violet
Newton's later colours	Red	Orange	Yellow	Green	Blue	Indigo	Violet
Modern interpretation	Red	Orange	Yellow	Green	Cyan	Blue	Violet

John Keats | Lamia, 1819

Do not all charms fly

At the mere touch of cold philosophy?

There was an awful rainbow once in heaven:

We know her woof, her texture; she is given

In the dull catalogue of common things.

Philosophy will clip an Angel's wings,

Conquer all mysteries by rule and line,

Empty the haunted air, and gnomed mine –

Unweave a rainbow.

In fisica dell'atmosfera e meteorologia, l'arcobaleno è un fenomeno atmosferico che produce uno spettro continuo di luce nel cielo quando la luce del Sole attraversa le gocce d'acqua rimaste in sospensione dopo un temporale, o presso una cascata od una fontana.

Lo spettro elettromagnetico dell'arcobaleno include lunghezze d'onda sia visibili sia non visibili all'occhio umano, queste ultime rilevabili attraverso uno spettrometro.